Loid is a great character to focus on. His double life as a spy and a father offers a lot of interesting story possibilities. You can explore his internal conflicts and how he manages to balance his dangerous job with his new family life.
Yor is another character worth focusing on. Her assassin side combined with her attempts to be a good wife and mother can create some really unique and engaging storylines. You could write about her trying to hide her true identity from the neighbors while also dealing with the everyday challenges of family life.
Loid is also a top pick. His stoic and secretive personality would make his reactions to unexpected situations or revelations in a react fanfic very engaging. He could be reacting to Anya's school life shenanigans or Yor's attempts at being a more 'normal' wife, and his internal struggle to maintain his cover while dealing with these emotions would be fascinating to explore.
Loid would also be an engaging character to center on. His spy skills and ability to adapt to any situation could be really cool in the Naruto world. He could use his disguise and infiltration skills to gather information about the ninja clans or even help Naruto on a mission. His calm and calculated nature would be a contrast to the more impulsive and emotional nature of some of the Naruto characters.
Beast himself is a great character to focus on. His intelligence and his struggle with his bestial form can lead to a lot of interesting storylines. For example, a story could explore his scientific research to find a cure for his mutation while also dealing with the moral implications of changing who he is.
The main family members can be great to focus on. Their relationships, power struggles, and dreams offer a lot of material. For example, the patriarch of the family who has to make tough decisions to keep the ranch going. Another good character could be the strong - willed matriarch who holds the family together. Also, the young and rebellious heir who wants to do things his own way.
Wolverine is always a great choice. His tough - guy attitude and healing factor make him adaptable in any crossover situation. He could hold his own against powerful characters from other universes. Another good one is Storm. Her control over the weather can be a game - changer in battles. In a crossover, she could use her powers to combat foes that others might struggle with.
From the Regeneration side, if there is a character with some sort of energy - based powers, they could be a good focus. Paired with Robin from Young Justice, they could create some really interesting strategies in battles. Another option could be a character who has a strong moral code from Regeneration, and when put together with Aqualad from Young Justice, they could have some great discussions about leadership and responsibility.
Well, any of the main characters really. But in addition to the ones mentioned, there could be a minor character that gets reborn. For example, one of Anya's classmates. A reborn classmate could have a different personality or set of skills. They could befriend Anya in a new way or cause some interesting plot developments. Maybe they have a hidden talent for hacking or something that gets revealed after being reborn and that could impact the overall story in the Spy x Family world.
The Medic could be a great addition. He could be the one patching up the Sniper and Spy when they get injured during their adventures. His medical knowledge and sometimes wacky personality could add some interesting comedic or tense moments. For example, when the Sniper is badly wounded, the Medic has to perform a risky procedure while the Spy is keeping watch and fending off potential threats.
Some popular platforms for fanfiction include Archive of Our Own (AO3) and FanFiction.net. AO3 has a wide range of Spy x Family fanfictions with various genres and themes. FanFiction.net also has a good collection. Just search for 'Spy x Family' on these sites, and you'll find many interesting stories.
All of the members of 4 town are great to focus on in different ways. Tae Young, for example, might be the subject of a fanfiction where his amazing dancing skills are the center of the story. Maybe he gets an offer to be in a big dance competition outside of the band, and he has to decide whether to take it or stay loyal to 4 town. Each member brings their own unique traits that can be explored in fanfiction.